The question as to their sleep, is one that is by no means difficult to solve. In the land animals, it is quite evident that all that have eyelids sleep. With reference to aquatic animals, it is admitted that they also sleep, though only for short periods, even by those writers who entertain doubts as to the other animals; and they come to this conclusion, not from any appearance of the eyes, for they have no eyelids, indeed, to close, but because they are to be seen buried in deep repose, and to all appearance fast asleep, betraying no motion in any part of the body except the tail, and by starting when they happen to hear a noise. With regard to the thunny, it is stated with still greater confidence that it sleeps; indeed, it is often found in that state near the shore, or among the rocks. Flat fish are also found fast asleep in shallow water, and are often taken in that state with the hand: and, as to the dolphin and the balæna, they are even heard to snore.

It is quite evident, also, that insects sleep, from the silent stillness which they preserve; and even if a light is put close to them, they will not be awoke thereby.